Generally, sealed beam lamp units have particular utility as head or auxiliary lamps for vehicles and comprise a glass lens which is hermetically sealed to a glass reflector at their mating peripheral surfaces to form a sealed lamp envelope. A portion of the glass envelope has a reflective coating applied thereto for reflecting visible radiation from a filament through a lens portion of the envelope to form a desired beam pattern. The filament is disposed within the envelope approximately at the focus thereof and is electrically connected by metal lead wires, to a set of lamp terminals which in turn are externally electrically connected to a power source.
Formerly, the rear lamp terminals were formed by embedding the rim of a cup-shaped ferrule in a heat-softened outer surface of a glass envelope boss to provide an hermetically sealed lamp envelope.
More recently, adhesive sealants such as epoxy resins or thermosetting cements have been used to seal lamp envelopes. However, unless the adhesive seal is substantially impervious to moisture penetration the lamp life is unacceptably short because water vapor attacks and degrades the reflectivity of the envelope, the transmissiveness of the lens and in the case of an exposed filament causes premature filament burnout due to the water cycle transport of tungsten from the hot filament to the cooler envelope wall.
Heretofore, adhesively sealed lamp constructions have employed mounting plates having light sources secured thereto which in turn are adhesively sealed in a rear reflector opening. Mounting brackets which are relatively heavy, strain the adhesives and create leak sites which permit moisture to penetrate the lamp.
The mounting plates indirectly position a light source and afford only limited focusing precision. The mounting plates have sockets for receiving a lamp therein or alternatively include channels for receiving lamp lead wires therethrough. Both the bulb-socket and the lead wire-channel constructions provide additional potential leak sites previously unknown in the cup-shaped ferrule-glass boss rear terminal constructions of the prior art.
Present technology suggests metal cladding of the adhesive seals with solder or sheet metal to create tortuous diffusion paths to guard against water vapor penetration. Metal cladding increases material and labor costs without reducing the number of leak sites or their respective cross-sectional areas.